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The nominal capacity is a characteristic value of batteries and accumulators. It expresses the current-carrying capacity in ampere seconds( As), ampere hours( Ah) or kilowatt hours (kWh) and is calculated from the product of charging current (I) in amperes (A) and charging time (t) in hours.

If a battery contains energy, a discharge current can be drawn over a certain period of time. The discharge current can be drawn until the final discharge voltage is reached. This applies equally to rechargeable batteries for mobile devices and to power batteries for electric vehicles. For example, if a mobile device battery has a rated capacity of 1,800 mAh, it can deliver a current of 200 mA for 9 hours or 100 mA for 18 hours. A battery of an electric vehicle with a nominal capacity of 50 kWh, can deliver a power of 20 kW for two hours and then has a residual energy of 10 kWh.

The duration for the discharge current is determined by the C coefficient, and the charge time by the charge factor. The dimensionless C-factor determines the time in which batteries deliver a constant discharge current. C values are given in a letter- digit notation, with a digit preceding or following the letter C. Examples: 1C, 2C, 5C, C/5, C/10. 1C means that a battery with a nominal capacity of 10 Ah, can supply one ampere for 10 hours. Fraction specifications are about the discharge time. For example, 0.2C or C/2 represents a five-hour discharge time with a discharge current of 200 mA.

The nominal capacity gives a theoretical value that cannot be maintained in practice, since rechargeable batteries always have a residual charge. This is determined according to the IEC standard IEC 61951-2. The remaining residual capacity of rechargeable batteries results from the nominal capacity, the percentage depth of discharge and the number of charge cycles.